the answer should be c.
because heat rises meaning hot air is less dense than cold air
Surface temperature of the star affects the color
So, the question is asking: why has (over the course of human history) rural settlement declined (and it is suggested that this is connected to humans moving away from the rural areas, to urban areas, rather than human population declining in general:
this process is called urbanization and its major cause is that on the one hand, food in the industrialized societies can be produced more effectively so there is less work on farm, and on the other hand the service sector has increased, which means that there is more work in the cities.
